This invention relates to an on-bottom seismometer system and, more particularly, to a system which will record seismic data and water break arrivals from repetitive seismic pulses.
T. J. G. Francis et al, Ocean Bottom Seismograph Marine Geophysical Researches (1975), 195-213, describe three ways of retrieving seismic information from the ocean floor. A self-contained instrument may be attached by a rope to a surface ship or buoy; a transducer package may telemeter its data to a surface ship buoy or land station for recording; and a completely independent instrument of the "pop up" type may record data in its self-contained memory. The latter type instrument has been referred to as an on-bottom seismometer. These instruments have the advantage that they are easier to place on a particular bottom feature; the instrument is more secure from being tampered with than one which is connected to a surface buoy; and less system generated noise is associated with this type of instrument than one employing a mooring or a telemetry link.
On-bottom seismometers have been extensively used to record explosion and earthquake data. Prothero, "A Free Fall Seismic Capsule for Seismicity and Refraction Work", Offshore Technology Conference Paper No. OTC2440, prepared for presentation at the 8th Annual Offshore Technology Conference, Houston, Texas, May 3-6, 1976; and, "An Operationally Optimized Ocean Bottom Seismometer Capsule", (Work sponsored by NOAA office of Sea Grant, under Grant 04-5-158-20), describe on-bottom seismometers which were developed at the University of California, Santa Barbara. In the former seismometer digital samples of seismic data are passed through a delaying shift register; in the latter seismometer they are stored in a microprocessor. When the amplitude of the signals is above the noise level in either seismometer, a tape recorder is started and the samples are transferred to magnetic tape.
Attempts to use on-bottom seismometers for refraction surveys have generally used a continuously running tape recorder to record the data. Avedik et al, "Ocean Bottom Refraction Seismograph (OBRS)", Marine Geophysical Researches 3 (1978), 357-379, describe an ocean bottom seismometer used for refraction surveys. In this system the seismic data are encoded with pulse width modulation. A tape recorder is set to start and stop by a programmer synchronized with the ship-borne seismic programmer. As best shown in FIG. 8, the recorder is running during the time that the seismic signal is being detected. U.S. Pat. No. 4,138,658, Avedik et al, discloses and claims some features of this unit.
Johnson et al, "A Direct Recording Ocean Bottom Seismometer", Marine Geophysical Researches 3 (1977), 65-85, describe another system using a continuously running tape recorder.
One problem associated with such tape recorders is the noise generated by the tape drive motor. If the tape recorder is running during the time that the seismic signals are being detected, this noise is mixed with the signal. Another problem with continuously running tape recorders is that only a limited number of seismograms can be recorded with good resolution. If the tape is run at very low speeds, more seismograms can be recorded, but the resolution is poor.
It is desirable to provide an ocean-bottom seismometer which can record a large number of refraction seismograms produced in response to repetitive shots of seismic energy. A system for producing repetitive pulses of seismic energy from air guns is shown, for example, in the Ritter U.S. Pat. No. 3,687,218.
It is desirable to record these refraction seismograms in the format of conventional reflection seismograms with header information and the time of arrival of the water break. The water break is the arrival of the direct traveling wave at the seismometer. This wave travels from the source to the ocean bottom through the water. The water break comes in later as the boat with the seismic source steams away from the seismometer. Because of this, prior art techniques for recording refraction seismograms including th water break, have required an exceedingly long record length. This limits the number of seismograms which can be recorded on a given magnetic tape.